Numerous chronic debilitating diseases of the skeletal system in vertebrates, including arthritis and related arthritic disorders, feature degradation of specialized avascular cartilaginous tissue known as articular cartilage that contains dedicated cartilage-producing cells, the articular chondrocytes. Unlike other chondrocytes such as epiphyseal growth plate chondrocytes present at the ends of developing long bones (e.g., endochondral or costochondral chondrocytes), articular chondrocytes reside in and maintain joint cartilage having no vasculature. Thus lacking a blood supply as an oxygen source, articular chondrocytes are believed to generate metabolic energy, for example bioenergetic ATP production, predominantly by anaerobic (e.g., glycolytic) respiration, and not from aerobic mitochondrial oxidative phosphorylation (Stefanovich-Racic et al., J. Cell Physiol. 159:274–80, 1994). Because even under aerobic conditions, articular chondrocytes may consume little oxygen and thus appear to differ from a wide variety of vertebrate cell types (Stefanoviceh-Racic et al., 1994), mitochondrial roles in arthritic disorders have been largely ignored.
The musculoskeletal system efficiently delivers useful mechanical energy and load support in vertebrates such as mammals, reptiles, birds and fish, but is also capable of synthesizing, processing and organizing complex macromolecules to fashion tissues and organs specialized to perform specific mechanical functions. The joints are an important subset of the specialized structures of the musculoskeletal system, and many distinct types of joints exist in the body. Freely moving joints (e.g., ankle, elbow, hip, knee, shoulder, and joints of the fingers, toes and wrist) are known as diarthrodial or synovial joints. In contrast, the intervertebral joints of the spine are not diarthrodial joints as they are fibrous and do not move freely, although they do provide the flexibility required by the spine. The articulating bone ends in the diarthrodial joint are lined with a thin layer of hydrated soft tissue known as articular cartilage. Fourth, the joint is stabilized by, and its range of motion controlled by, ligaments and tendons that may be inside or outside the joint capsule.
The surface linings of diarthrodial joints, i.e., the synovium and articular cartilage layers, form the joint cavity that contains the synovial fluid. Thus, in vertebrate skeletal joints, the synovial fluid, articular cartilage, and the supporting bone form a smooth, nearly frictionless bearing system. While diarthrodial joints are subjected to an enormous and varied range of load conditions, the cartilage surfaces undergo little wear and tear (e.g., structural degradation) under normal circumstances. Indeed, most human joints are capable of functioning effectively under very high loads and stresses and at very low operating speeds. These performance characteristics demand efficient lubrication processes to minimize friction and wear of cartilage in the joint. Severe breakdown of the joint cartilage by biochemical and/or biomechanical processes leads to arthritis, which is therefore generally defined as a failure of the vertebrate weight bearing system.
Articular chondrocytes synthesize and deposit the components of, and reside in, a three-dimensional cartilaginous extracellular matrix comprised largely of two major classes of macromolecules, collagen and proteoglycans. Articular chondrocytes thus mediate the synthesis, assembly, degradation and turnover of the macromolecules which comprise the cartilage extracellular matrix (ECM or simply “matrix”). Mechanochemical properties of this matrix contribute significantly to the biomechanical function of cartilage in vivo.
The structural integrity of articular cartilage is the foundation of optimal functioning of the skeletal joints, such as those found in the vertebrate hip, shoulders, elbows, hocks and stifles. Impaired skeletal joint function dramatically reduces an individual subject's mobility, such as that involved in rising from a sitting position or in climbing and descending stairs. As noted above, in order to maintain the structural and functional integrity of articular cartilage, articular chondrocytes constantly synthesize collagen and proteoglycans, the major components of the articular cartilage; chondrocytes also secrete the friction-reducing synovial fluid. This constant elaboration by articular chondrocytes of cartilage ECM macromolecules and synovial fluid provides the articular cartilage with a repair mechanism for most mechanical wear that may be caused by friction between the bone ends. However, such steady biosynthesis of cartilage components generates a constant demand for the precursors, or building blocks, of these macromolecules and synovial fluid components. Lack of these precursors will lead to defects in the structure and function of the skeletal joints. This deficiency occurs often when activity levels are very high, or when cartilage tissue is traumatized.
The menisci of the knee, and other similar structures such as the disc of the temporomandibular joint and the labrum of the shoulder, are specialized fibrocartilagenous structures that are vital for normal joint function. They are known to assist articular cartilage in distributing loads across the joint, to aid ligaments and tendons in stabilizing joints and to play a major role in shock absorption, and may further assist in lubricating the joint. Damage to these structures can lead to impaired joint function and to articular cartilage degeneration. Surgical removal of these fibrocartilagenous structures, for example, following apparently irreparable cartilage tears, can result in early onset of osteoarthritis. The menisci, disc and labrum are hydrated fibrocartilage structures composed primarily of type II collagen, with smaller amounts of other collagens and proteoglycans (including aggrecan and the smaller, non-aggregating proteoglycans) also present. These fibrocartilaginous structures contain a sparse population of resident cells that, like the articular chondrocytes of cartilage, are responsible for the synthesis and maintenance of this extracellular matrix.
Diarthrodial joints enable common bodily motions including limb movements associated with motor (e.g., ambulatory) functions and other activities of daily life. Failure of the joint surfaces (i.e., articular cartilage) means a failure of these biomechanical bearings to provide their central functions, such as ambulatory and other bodily motion, delivery of mechanical energy and load support.
In biomedical terms, failure of diarthrodial joints leads to arthritic disorders, the most common forms being osteoarthritis or degenerative joint disease, or chondrocalcinosis. Other forms of arthritic disorders include but are not limited to rheumatoid arthritis, juvenile rheumatoid arthritis, ankylosing spondylitis, Reiter's syndrome, psoriatic arthritis, lupus erythematosous, gout, infectious arthritides and chondrocalcinosis (see, e.g., Gilliland et al., “Disorders of the joints and connective tissue,” Section 14, Harrison's Principles of Internal Medicine, Eighth Ed., Thorn et al., eds. McGraw-Hill, New York, N.Y., 1977, pp. 2048–80) and, in a veterninary context, dysplasias such as canine hip dysplasia. Arthritic disorders can also include, or may result from, physical trauma (for example, acute physical injury that damages joint tissue, or repetitive motion syndrome) or dietary conditions (e.g., ricketts or other dietary deficiency diseases) that result in joint injury.
By far, the most prevalent arthritic disorders are rheumatoid arthritis (RA) and osteoarthritis (OA). RA, thought to be an autoimmune disorder, results in part from inflammation of the synovial membrane. In humans, peak onset of this disorder occurs in adults over 30 years of age (typically in their thirties and forties) and afflicts women three times more often than men. In extreme cases, chronic inflammation erodes and distorts the joint surfaces and connective tissue, resulting in severe articular deformity and constant pain. Moreover, RA often leads to OA, further compounding the destruction of the joint. The most common arthritic disorder, OA, is characterized by degenerative changes in the surface of the articular cartilage. Alterations in the physicochemical structure of the cartilage make it less resistant to compressive and tensile forces. Finally, complete erosion occurs, leaving the subchondral bone exposed and susceptible to wear. Joints of the knees and hands are most often affected, as also may be one or more of the spine, hips, ankles and shoulders. In both RA and OA, degeneration of the weight bearing joints such as the hips and knees can be especially debilitating and often requires surgery to relieve pain, and to increase mobility.
No means currently exist for halting or reversing the degenerative changes brought about by RA, OA and related arthritic disorders. At the same time, approximately 37 million Americans seek symptomatic relief in the form of prescription drugs. In such cases nonsteroidal, anti-inflammatory drugs (NSAIDS) are most often prescribed. While these compounds often alleviate or palliate the arthritic symptoms, they frequently have undesirable side effects, for example, nausea and gastrointestinal ulceration. Other compounds commonly prescribed for the treatment of arthritic disorders are the corticosteroids, such as triamcinolone, prednisolone and hydrocortisone. These drugs also have undesirable side effects, particularly where long term use may be required, and so may be contraindicated in many patients. In addition to difficulties in determining effective dosages, a number of adverse reactions have been reported during intra-articular treatment with these and other steroids. As a result, the use of corticosteroid treatments in the management of arthritic disorders is currently being reassessed.
As an alternative to symptomatic and palliative measures for treating OA and RA as described above, mechanical repair of arthritic joints, when feasible, involves orthopedic surgery to replace worn joints with an artificial prosthesis, or with a biological graft. With more than thirty million Americans suffering from these disabling diseases, such surgery poses enormous medical and economic challenges and is not without its own risks and contraindications.
As noted above, osteoarthritis, also known as degenerative joint disease, is one of the most common types of arthritis. It is characterized by the breakdown of the cartilage within a joint, causing painful rubbing of one bone of the joint against another bone and leading to a loss of movement within the affected joint. Osteoarthritis can range from very mild to very severe, and most commonly affects middle-aged and older people. In particular, OA often affects hands and weight-bearing joints such as the knees, hips, feet and back. Although age is a leading risk factor, at present the etiology and pathogenesis of this condition remain largely unknown. Many environmental factors and other independent conditions have been associated with an increased risk for having or developing osteoarthritis, including obesity, previous injury and/or menisectomy (e.g., sports-related injuries or other accidental injury), occupation-related activities that involve repeated knee bending, smoking, sex hormones, gynecological disorders and other metabolic factors. Chondrocalcinosis is another form of degenerative joint disease related to osteoarthritis, in which abnormal calcification occurs in the articular cartilage.
From the foregoing, it is clear that none of the current pharmacological therapies corrects the underlying biochemical defect in arthritic disorders such as RA and OA. Neither do any of these currently available treatments improve all of the physiological abnormalities in arthritic disorders such as abnormal articular chondrocyte activity, cartilage degradation, articular erosion and severe joint deformity. In addition, treatment failures are common with these agents, such that multi-drug therapy is frequently necessary.
Clearly there is a need for improved therapeutics that are targeted to correct biochemical and/or metabolic defects responsible for arthritis. The present invention provides compositions and methods that are useful for treating an arthritic disorder and for treating other diseases, and offers other related advantages.
According to non-limiting theory, and as disclosed in the co-pending application having U.S. Ser. No. 09/661,848, which is incorporated by reference, some or all arthritic disorders as provided herein may represent examples of diseases associated with altered mitochondrial function.
By way of background, mitochondria are the main energy source in cells of higher organisms, and these organelles provide direct and indirect biochemical regulation of a wide array of cellular respiratory, oxidative and metabolic processes (for a review, see Ernster and Schatz, J. Cell Biol. 91:227s–255s, 1981). These include electron transport chain (ETC) activity, which drives oxidative phosphorylation to produce metabolic energy in the form of adenosine triphosphate (ATP), and which also underlies a central mitochondrial role in intracellular calcium homeostasis. In addition to their role in metabolic processes, mitochondria are also involved in the genetically programmed cell suicide sequence known as “apoptosis” (Green and Reed, Science 281:1309–12, 1998; Susin et al., Biochim. et Biophys. Acta 1366:151–65, 1998).
Defective mitochondrial activity, including but not limited to failure at any step of the elaborate multi-complex mitochondrial assembly, known as the electron transport chain (ETC), may result in (i) decreases in ATP production, (ii) increases in the generation of highly reactive free radicals (e.g., superoxide, peroxynitrite and hydroxyl radicals, and hydrogen peroxide), (iii) disturbances in intracellular calcium homeostasis and (iv) the release of factors (such as such as cytochrome c and “apoptosis inducing factor”) that initiate or stimulate the apoptosis cascade. Because of these biochemical changes, mitochondrial dysfunction has the potential to cause widespread damage to cells and tissues.
A number of diseases and disorders are thought to be caused by or be associated with alterations in mitochondrial metabolism and/or inappropriate induction or suppression of mitochondria-related functions, such as those leading to apoptosis. These include, by way of example and not limitation, chronic neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD); auto-immune diseases; diabetes mellitus, including Type I and Type II; mitochondria associated diseases, including but not limited to congenital muscular-dystrophy with mitochondrial structural abnormalities, fatal infantile myopathy with severe mtDNA depletion and benign “later-onset” myopathy with moderate reduction in mtDNA, MELAS (mitochondrial encephalopathy, lactic acidosis, and stroke) and MIDD (mitochondrial diabetes and deafness); MERFF (myoclonic epilepsy ragged red fiber syndrome); arthritis; NARP (Neuropathy; Ataxia; Retinitis Pigmentosa); MNGIE (Myopathy and external ophthalmoplegia; Neuropathy; Gastro-Intestinal; Encephalopathy), LHON (Leber's; Hereditary; Optic; Neuropathy), Kearns-Sayre disease; Pearson's Syndrome; PEO (Progressive External Ophthalmoplegia); Wolfram syndrome DIDMOAD (Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, Deafness); Leigh's Syndrome; dystonia; schizophrenia; and hyperproliferative disorders, such as cancer, tumors and psoriasis. The extensive list of additional diseases associated with altered mitochondrial function continues to expand as aberrant mitochondrial or mitonuclear activities are implicated in particular disease processes.
According to generally accepted theories of mitochondrial function, proper ETC respiratory activity requires maintenance of an electrochemical potential (ΔΨm) in the inner mitochondrial membrane by a coupled chemiosmotic mechanism. Conditions that dissipate or collapse this membrane potential, including but not limited to failure at any step of the ETC, may thus prevent ATP biosynthesis and hinder or halt the production of a vital biochemical energy source. Altered or defective mitochondrial activity may also result in a catastrophic mitochondrial collapse that has been termed “mitochondrial permeability transition” (MPT). In addition, mitochondrial proteins such as cytochrome c and “apoptosis inducing factor” may dissociate or be released from mitochondria due to MPT (or the action of mitochondrial proteins such as Bax), and may induce proteases known as caspases and/or stimulate other events in apoptosis (Murphy, Drug Dev. Res. 46:18–25, 1999).
Defective mitochondrial activity may alternatively or additionally result in the generation of highly reactive free radicals that have the potential of damaging cells and tissues. These free radicals may include reactive oxygen species (ROS) such as superoxide, peroxynitrite and hydroxyl radicals, and potentially other reactive species that may be toxic to cells. For example, oxygen free radical induced lipid peroxidation is a well established pathogenetic mechanism in central nervous system (CNS) injury such as that found in a number of degenerative diseases, and in ischemia (i.e., stroke). (Mitochondrial participation in the apoptotic cascade is believed to also be a key event in the pathogenesis of neuronal death.)
There are, moreover, at least two deleterious consequences of exposure to reactive free radicals arising from mitochondrial dysfunction that adversely impact the mitochondria themselves. First, free radical mediated damage may inactivate one or more of the myriad proteins of the ETC. Second, free radical mediated damage may result in catastrophic mitochondrial collapse that has been termed “transition permeability”. According to generally accepted theories of mitochondrial function, proper ETC respiratory activity requires maintenance of an electrochemical potential in the inner mitochondrial membrane by a coupled chemiosmotic mechanism. Free radical oxidative activity may dissipate this membrane potential, thereby preventing ATP biosynthesis and/or triggering mitochondrial events in the apoptotic cascade. Therefore, by modulating these and other effects of free radical oxidation on mitochondrial structure and function, the present invention provides compositions and methods for protecting mitochondria that are not provided by the mere determination of free radical induced lipid peroxidation.
For example, rapid mitochondrial permeability transition likely entails changes in the inner mitochondrial transmembrane protein adenylate translocase that results in the formation of a “pore.” Whether this pore is a distinct conduit or simply a widespread leakiness in the membrane is unresolved. In any event, because permeability transition is potentiated by free radical exposure, it may be more likely to occur in the mitochondria of cells from patients having mitochondria associated diseases that are chronically exposed to such reactive free radicals.
Altered (e.g., increased or decreased in a statistically significant manner relative to an appropriate control, such as a disease-free individual) mitochondrial function characteristic of the mitochondria associated diseases may also be related to loss of mitochondrial membrane electrochemical potential by mechanisms other than free radical oxidation, and such transition permeability may result from direct or indirect effects of mitochondrial genes, gene products or related downstream mediator molecules and/or extramitochondrial genes, gene products or related downstream mediators, or from other known or unknown causes. Loss of mitochondrial potential therefore may be a critical event in the progression of mitochondria associated or degenerative diseases.
Diabetes mellitus is a common, degenerative disease affecting 5 to 10 percent of the population in developed countries. The propensity for developing diabetes mellitus is reportedly maternally inherited, suggesting a mitochondrial genetic involvement. (Alcolado, J. C. and Alcolado, R., Br. Med. J. 302:1178–80, 1991; Reny, S. L., International J. Epidem. 23:886–90, 1994.) Diabetes is a heterogenous disorder with a strong genetic component; monozygotic twins are highly concordant and there is a high incidence of the disease among first-degree relatives of affected individuals.
At the cellular level, the degenerative phenotype that may be characteristic of late onset diabetes mellitus includes indicators of altered mitochondrial respiratory function, for example impaired insulin secretion, decreased ATP synthesis and increased levels of reactive oxygen species. Studies have shown that diabetes mellitus may be preceded by or associated with certain related disorders. For example, it is estimated that forty million individuals in the U.S. suffer from late onset impaired glucose tolerance (IGT). IGT patients fail to respond to glucose with increased insulin secretion. A small percentage of IGT individuals (5–10%) progress to insulin deficient non-insulin dependent diabetes (NIDDM) each year. Some of these individuals further progress to insulin dependent diabetes mellitus (IDDM). These forms of diabetes mellitus, NIDDM and IDDM, are associated with decreased release of insulin by pancreatic beta cells and/or a decreased end-organ response to insulin. Other symptoms of diabetes mellitus and conditions that precede or are associated with diabetes mellitus include obesity, vascular pathologies, peripheral and sensory neuropathies, blindness and deafness.
Due to the strong genetic component of diabetes mellitus, the nuclear genome has been the main focus of the search for causative genetic mutations. However, despite intense effort, nuclear genes that segregate with diabetes mellitus are known only for rare mutations in the insulin gene, the insulin receptor gene, the adenosine deaminase gene and the glucokinase gene. Accordingly, mitochondrial defects, which may include but need not be limited to defects related to the discrete non-nuclear mitochondrial genome that resides in mitochondrial DNA, may contribute significantly to the pathogenesis of diabetes mellitus (Anderson, Drug Dev. Res. 46:67–79, 1999).
Parkinson's disease (PD) is a progressive, chronic, mitochondria-associated neurodegenerative disorder characterized by the loss and/or atrophy of dopamine-containing neurons in the pars compacta of the substantia nigra of the brain. Like Alzheimer's Disease (AD), PD also afflicts the elderly. It is characterized by bradykinesia (slow movement), rigidity and a resting tremor. Although L-Dopa treatment reduces tremors in most patients for a while, ultimately the tremors become more and more uncontrollable, making it difficult or impossible for patients to even feed themselves or meet their own basic hygiene needs.
It has been shown that the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces parkinsonism in animals and man at least in part through its effects on mitochondria. MPTP is converted to its active metabolite, MPP+, in dopamine neurons; it then becomes concentrated in the mitochondria. The MPP+ then selectively inhibits the mitochondrial enzyme NADH:ubiquinone oxidoreductase (“Complex I”), leading to the increased production of free radicals, reduced production of adenosine triphosphate, and ultimately, the death of affected dopamine neurons.
Mitochondrial Complex I is composed of 40–50 subunits; most are encoded by the nuclear genome and seven by the mitochondrial genome. Since parkinsonism may be induced by exposure to mitochondrial toxins that affect Complex I activity, it appears likely that defects in Complex I proteins may contribute to the pathogenesis of PD by causing a similar biochemical deficiency in Complex I activity. Indeed, defects in mitochondrial Complex I activity have been reported in the blood and brain of PD patients (Parker et al., Am. J. Neurol. 26:719–23, 1989; Swerdlow and Parker, Drug Dev. Res. 46:44–50, 1999).
Similar theories have been advanced for analogous relationships between mitochondrial defects and other neurological diseases, including Alzheimer's disease, Leber's hereditary optic neuropathy, schizophrenia, “mitochondrial encephalopathy, lactic acidosis, and stroke” (MELAS), and “myoclonic epilepsy ragged red fiber syndrome” (MERRF).
For example, Alzheimer's disease (AD) is a chronic, progressive neurodegenerative disorder that is characterized by loss and/or atrophy of neurons in discrete regions of the brain, and that is accompanied by extracellular deposits of β-amyloid and the intracellular accumulation of neurofibrillary tangles. It is a uniquely human disease, affecting over 13 million people worldwide. It is also a uniquely tragic disease. Many individuals who have lived normal, productive lives are slowly stricken with AD as they grow older, and the disease gradually robs them of their memory and other mental faculties. Eventually, they cease to recognize family and loved ones, and they often require continuous care until their eventual death.
There is evidence that defects in oxidative phosphorylation within the mitochondria are at least a partial cause of sporadic AD. The enzyme cytochrome c oxidase (COX), which makes up part of the mitochondrial electron transport chain (ETC), is present in normal amounts in AD patients; however, the catalytic activity of this enzyme in AD patients and in the brains of AD patients at autopsy has been found to be abnormally low. This suggests that the COX in AD patients is defective, leading to decreased catalytic activity that in some fashion causes or contributes to the symptoms that are characteristic of AD.
One hallmark pathology of AD is the death of selected neuronal populations in discrete regions of the brain. Cell death in AD is presumed to be apoptotic because signs of programmed cell death (PCD) are seen and indicators of active gliosis and necrosis are not found (Smale et al., Exp. Neurolog. 133:225–30, 1995; Cotman et al., Molec. Neurobiol. 10:19–45, 1995.) The consequences of cell death in AD, neuronal and synaptic loss, are closely associated with the clinical diagnosis of AD and are highly correlated with the degree of dementia in AD (DeKosky et al., Ann. Neurol. 27(5):467–64, 1990).
Mitochondrial dysfunction is thought to be critical in the cascade of events leading to apoptosis in various cell types (Kroemer et al., FASEB J. 9:1277–87, 1995), and may be a cause of apoptotic cell death in neurons of the AD brain. Altered mitochondrial physiology may be among the earliest events in PCD (Zamzami et al., J. Exp. Med 182:367–77, 1995; Zamzami et al., J. Exp. Med. 181:1661–72, 1995) and elevated reactive oxygen species (ROS) levels that result from such altered mitochondrial function may initiate the apoptotic cascade (Ausserer et al., Mol. Cell. Biol. 14:5032–42, 1994). In several cell types, including neurons, reduction in the mitochondrial membrane potential (ΔΨm) precedes the nuclear DNA degradation that accompanies apoptosis. In cell-free systems, mitochondrial, but not nuclear, enriched fractions are capable of inducing nuclear apoptosis (Newmeyer et al., Cell 70:353–64, 1994). Perturbation of mitochondrial respiratory activity leading to altered cellular metabolic states, such as elevated intracellular ROS, may occur in mitochondria associated diseases and may further induce pathogenetic events via apoptotic mechanisms.
Oxidatively stressed mitochondria may release a pre-formed soluble factor that can induce chromosomal condensation, an event preceding apoptosis (Marchetti et al., Cancer Res. 56:2033–38, 1996). In addition, members of the Bcl-2 family of anti-apoptosis gene products are located within the outer mitochondrial membrane (Monaghan et al., J. Histochem. Cytochem. 40:1819–25, 1992) and these proteins appear to protect membranes from oxidative stress (Korsmeyer et al, Biochim. Biophys. Act. 1271:63, 1995). Localization of Bcl-2 to this membrane appears to be indispensable for modulation of apoptosis (Nguyen et al., J. Biol. Chem. 269:16521–24, 1994). Thus, changes in mitochondrial physiology may be important mediators of apoptosis. To the extent that apoptotic cell death is a prominent feature of neuronal loss in AD, mitochondrial dysfunction may be critical to the progression of this disease and may also be a contributing factor in other mitochondria associated diseases.
Focal defects in energy metabolism in the mitochondria, with accompanying increases in oxidative stress, may be associated with AD. It is well-established that energy metabolism is impaired in AD brain (Palmer et al., Brain Res. 645:338–42, 1994; Pappolla et al., Am. J. Pathol. 140:621–28, 1992; Jeandel et al., Gerontol. 35:275, 1989; Balazs et al., Neurochem. Res. 19:1131–37, 1994; Mecocci et al., Ann. Neurol. 36:747–51, 1994; Gsell et al., J. Neurochem. 64:1216–23, 1995). For example, regionally specific deficits in energy metabolism in AD brains have been reported in a number of positron emission tomography studies (Kuhl, et al., J. Cereb. Blood Flow Metab. 7:S406, 1987; Grady, et al., J. Clin. Exp. Neuropsychol. 10:576–96, 1988; Haxby et al., Arch. Neurol. 47:753–60, 1990; Azari et al., J. Cereb. Blood Flow Metab. 13:438–47, 1993). Metabolic defects in the temporoparietal neocortex of AD patients apparently presage cognitive decline by several years. Skin fibroblasts from AD patients display decreased glucose utilization and increased oxidation of glucose, leading to the formation of glycosylation end products (Yan et al., Proc. Nat. Acad. Sci. U.S.A. 91:7787–91, 1994). Cortical tissue from postmortem AD brain shows decreased activity of the mitochondrial enzymes pyruvate dehydrogenase (Sheu et al., Ann. Neurol. 17:444–49, 1985) and α-ketoglutarate dehydrogenase (Mastrogiacomo et al., J. Neurochem. 6:2007–14, 1994), which are both key enzymes in energy metabolism. Functional magnetic resonance spectroscopy studies have shown increased levels of inorganic phosphate relative to phosphocreatine in AD brain, suggesting an accumulation of precursors that arises from decreased ATP production by mitochondria (Pettegrew et al., Neurobiol. of Aging 15:117–32, 1994; Pettigrew et al., Neurobiol. of Aging 16:973–75, 1995). In addition, the levels of pyruvate, but not of glucose or lactate, are reported to be increased in the cerebrospinal fluid of AD patients, consistent with defects in cerebral mitochondrial electron transport chain (ETC) activity (Parnetti et al., Neurosci. Lett. 199:231–33, 1995).
Signs of oxidative injury are prominent features of AD pathology and, as noted above, reactive oxygen species (ROS) are critical mediators of neuronal degeneration. Indeed, studies at autopsy show that markers of protein, DNA and lipid peroxidation are increased in AD brain (Palmer et al., Brain Res. 645:338–42, 1994; Pappolla et al., Am. J. Pathol. 140:621–28, 1992; Jeandel et al., Gerontol. 35:275–82, 1989; Balazs et al., Arch. Neurol. 4:864, 1994; Mecocci et al., Ann. Neurol. 36:747–51, 1994; Smith et al., Proc. Nat. Acad. Sci. U.S.A. 88:10540–43, 1991). In hippocampal tissue from AD but not from controls, carbonyl formation indicative of protein oxidation is increased in neuronal cytoplasm, and nuclei of neurons and glia (Smith et al., Nature 382:120–21, 1996). Neurofibrillary tangles also appear to be prominent sites of protein oxidation (Schweers et al., Proc. Nat. Acad. Sci. U.S.A. 92:8463, 1995; Blass et al., Arch. Neurol. 4:864, 1990). Under stressed and non-stressed conditions incubation of cortical tissue from AD brains taken at autopsy demonstrate increased free radical production relative to non-AD controls. In addition, the activities of critical antioxidant enzymes, particularly catalase, are reduced in AD (Gsell et al., J. Neurochem. 64:1216–23, 1995), suggesting that the AD brain is vulnerable to increased ROS production. Thus, oxidative stress may contribute significantly to the pathology of mitochondria associated diseases such as AD, where mitochondrial dysfunction and/or elevated ROS may be present.
Increasing evidence points to the fundamental role of mitochondrial dysfunction in chronic neurodegenerative diseases (Beal, Biochim. Biophys. Acta 1366:211–23, 1998), and recent studies implicate mitochondria for regulating the events that lead to necrotic and apoptotic cell death (Susin et al., Biochim. Biophys. Acta 1366:151–68, 1998). Stressed (by, e.g., free radicals, high intracellular calcium, loss of ATP, among others) mitochondria may release pre-formed soluble factors that can initiate apoptosis through an interaction with apoptosomes (Marchetti et al., Cancer Res. 56:2033–38, 1996; Li et al., Cell 91:479–89, 1997). Release of preformed soluble factors by stressed mitochondria, like cytochrome c, may occur as a consequence of a number of events. In any event, it is thought that the magnitude of stress (ROS, intracellular calcium levels, etc.) influences the changes in mitochondrial physiology that ultimately determine whether cell death occurs via a necrotic or apoptotic pathway. To the extent that apoptotic cell death is a prominent feature of degenerative diseases, mitochondrial dysfunction may be a critical factor in disease progression.
In contrast to chronic neurodegenerative diseases, neuronal death following stroke occurs in an acute manner. A vast amount of literature now documents the importance of mitochondrial function in neuronal death following ischemia/reperfusion injury that accompanies stroke, cardiac arrest and traumatic injury to the brain. Experimental support continues to accumulate for a central role of defective energy metabolism, alteration in mitochondrial function leading to increased oxygen radical production and impaired intracellular calcium homeostasis, and active mitochondrial participation in the apoptotic cascade in the pathogenesis of acute neurodegeneration.
A stroke occurs when a region of the brain loses perfusion and neurons die acutely or in a delayed manner as a result of this sudden ischemic event. Upon cessation of the blood supply to the brain, tissue ATP concentration drops to negligible levels within minutes. At the core of the infarct, lack of mitochondrial ATP production causes loss of ionic homeostasis, leading to osmotic cell lysis and necrotic death. A number of secondary changes can also contribute to cell death following the drop in mitochondrial ATP. Cell death in acute neuronal injury radiates from the center of an infarct where neurons die primarily by necrosis to the penumbra where neurons undergo apoptosis to the periphery where the tissue is still undamaged (Martin et al., Brain Res. Bull. 46:281–309, 1998).
Much of the injury to neurons in the penumbra is caused by excitotoxicity induced by glutamate released during cell lysis at the infarct focus, especially when exacerbated by bioenergetic failure of the mitochondria from oxygen deprivation (MacManus and Linnik, J. Cerebral Blood Flow Metab. 17:815–32, 1997). The initial trigger in excitotoxicity is the massive influx of Ca2+ primarily through the NMDA receptors, resulting in increased uptake of Ca2+ into the mitochondria (reviewed by Dykens, “Free radicals and mitochondrial dysfunction in excitotoxicity and neurodegenerative diseases” in Cell Death and Diseases of the Nervous System, V. E. Koliatos and R. R. Ratan, eds., Humana Press, New Jersey, pp. 45–68, 1999). The Ca2+ overload collapses the mitochondrial membrane potential (ΔΨm) and induces increased production of reactive oxygen species (Dykens, J Neurochem 63:584–91, 1994; Dykens, “Mitochondrial radical production and mechanisms of oxidative excitotoxicity” in The Oxygen Paradox, K. J. A. Davies, and F. Ursini, eds., Cleup Press, U. of Padova, pages 453–67, 1995). If severe enough, ΔΨm collapse and mitochondrial Ca2+ sequestration can induce opening of a pore in the inner mitochondrial membrane through a process called mitochondrial permeability transition (MPT), indirectly releasing cytochrome c and other proteins that initiate apoptosis (Bernardi et al., J. Biol. Chem. 267:2934–39, 1994; Zoratti and Szabo, Biochim. Biophys. Acta 1241:139–76, 1995; Ellerby et al., J Neurosci 17:6165–78, 1997). Consistent with these observations, glutamate-induced excitotoxicity can be inhibited by preventing mitochondrial Ca2+ uptake or blocking MPT (Budd and Nichols, J Neurochem 66:403–11, 1996; White and Reynolds, J Neurosci 16:5688–97, 1996; Li et al., Brain Res. 753:133–40, 1997).
Whereas mitochondria-mediated apoptosis may be critical in degenerative diseases, it is thought that disorders such as cancer involve the unregulated and undesirable growth (hyperproliferation) of cells that have somehow escaped a mechanism that normally triggers apoptosis in such undesirable cells. Enhanced expression of the anti-apoptotic protein, Bcl-2 and its homologues is involved in the pathogenesis of numerous human cancers. Bcl-2 acts by inhibiting programmed cell death and overexpression of Bcl-2, and the related protein Bcl-xL, block mitochondrial release of cytochrome c from mitochondria and the activation of caspase 3 (Yang et al, Science 275:1129–32, 1997; Kluck et al., Science 275:1132–36, 1997; Kharbanda et al., Proc. Natl. Acad. Sci. USA. 94:6939–42, 1997). In contrast, overexpression of Bcl-2 and Bcl-xL protect against the mitochondrial dysfunction preceding nuclear apoptosis that is induced by chemotherapeutic agents. In addition, acquired multi-drug resistance to cytotoxic drugs is associated with inhibition cytochrome c release that is dependent on overexpression of Bcl-xL (Kojima et al., J. Biol. Chem. 273:16647–50, 1998). Because mitochondria have been implicated in apoptosis, it is expected that agents that interact with mitochondrial components will effect a cell's capacity to undergo apoptosis. Thus, agents that induce or promote apoptosis in hyperproliferative cells are expected to be useful in treating hyperproliferative disorders and diseases such as cancer.
Thus, alteration of mitochondrial function has great potential for a broad-based therapeutic strategy for designing drugs to treat diseases associated with altered mitochondrial function, including (by way of non-limiting theory) certain arthritic disorders, degenerative disorders and hyperproliferative diseases. Further according to non-limiting theory, depending on the disease or disorder for which treatment is sought, such drugs may be, for example, mitochondria protecting agents, anti-apoptotic agents or pro-apoptotic agents.
Clearly there is a need for compounds and methods that limit or prevent damage to organelles, cells and tissues that results directly or indirectly from mitochondrial dysfunction, for example damage by free radicals generated intracellularly. In particular, because mitochondria are essential organelles for producing metabolic energy, agents that protect mitochondria against such damage (e.g., oxidative injury by free radicals) would be especially useful. Such agents may be suitable for the treatment of degenerative diseases including mitochondria associated diseases. Existing approaches to identifying agents that limit oxidative damage may not include determination of whether such agents may help protect mitochondrial structure and/or function.
There is also a need for compounds and methods that limit or prevent damage to cells and tissues that occurs directly or indirectly as a result of necrosis and/or inappropriate apoptosis. In particular, because mitochondria are mediators of apoptotic events, agents that modulate mitochondrially mediated pro-apoptotic events would be especially useful. Such agents may be suitable for the treatment of acute degenerative events such as stroke. Given the limited therapeutic window for blockade of necrotic death at the core of an infarct, it may be particularly desirable to develop therapeutic strategies to limit neuronal death by preventing mitochondrial dysfunction in the non-necrotic regions of an infarct. Agents and methods that maintain mitochondrial integrity during transient ischemia and the ensuing wave of excitotoxicity would be expected to be novel neuroprotective agents with utility in limiting stroke-related neuronal injury.
There is also a need for compounds and methods that inhibit the growth or enhance the death of cells and tissues that have escaped appropriate apoptotic signals, as well as cytotoxic agents that cause the death of undesirable (e.g., cancer) cells by triggering the apoptotic cascade. In particular, because mitochondria are mediators of apoptotic events, agents that stimulate mitochondrially mediated pro-apoptotic events would be especially useful. Such agents may be suitable for the treatment of hyperproliferative diseases such as cancer and psoriasis.
The present invention fulfills these needs and provides other related advantages. Those skilled in the art will recognize further advantages and benefits of the invention after reading the disclosure.